The invention relates to an exhaust gas afterburner device for internal combustion engines. More specifically it relates to a hollow plate type fitment for injecting a plurality of streams of fresh air laterally into each exhaust gas stream through a thin planar gasket area of the fitment located between the engine block and its exhaust manifold. Fresh air is supplied under pressure to the gasket section through an integrally attached thin plenum section extending along the underside or bottom edge of the gasket section. The injection of fresh air through a plurality of input openings surrounding a substantial portion of each exhaust gas stream provides improved afterburning over comparable prior art systems which inject air into each exhaust gas stream through a single input opening. A number of other significant advantages provided by this invention relate to the fact that the fresh air input passages are spread over a relatively large area of the gasket section surrounding each exhaust port. The fresh air is preheated efficiently as it flows through relatively narrow passages disposed within the gasket section and in doing so cools a portion of the gasket section thus providing a partial thermal barrier at the interface of the engine block and manifold. Some prior preheating also occurs in the thin plenum section because of its location in close proximity to the hot exhaust manifold. Additionally installation is simplified because the afterburner fitment replaces the manifold gasket and does not require multiple tubing connections.